1. Field of the Invention
The present invention relates to an imaging apparatus and a method for controlling the imaging apparatus. More particularly, the present invention relates to an imaging apparatus capable of capturing an image with reduced light falloff at edges and a method for controlling the same.
2. Description of the Related Art
An imaging apparatus is configured to capture an image of an object via a lens. The amount of light is relatively low in the peripheral region of a captured image compared to that in the central region. This is referred to as “light falloff at edges.”
Light falloff at edges is generally dependent on a relationship between the properties of a lens equipped in an imaging apparatus and properties of a solid-state image sensor.
More specifically, the amount of light falloff at edges is variable according to a positional relationship between the solid-state image sensor and an effective image circle (the diameter of the sharp image circle formed by a lens). In this case, an effective pixel center of the solid-state image sensor accords with an optical axis center of the lens.
Furthermore, the amount of light falloff at edges is variable according to an aperture value of the lens. Light falloff is maximized when the aperture is fully opened. According to general lens properties, incident light passing through the center of a lens has a circular light flux, while incident light passing through a peripheral region of the lens has an elliptic light flux. Thus, the amount of light decreases at the peripheral region of the lens.
When the diaphragm is in a full aperture state, a relatively large amount of incident light passes through a peripheral region of the lens. The shading of light greatly increases. Therefore, light falloff at edges greatly increases.
The above-described phenomenon can be eliminated if a lens equipped in an imaging apparatus has an effective image circle that is sufficiently large compared to the number of pixels provided in the solid-state image sensor. However, a lens satisfying such conditions has a large size. The imaging apparatus cannot be downsized if the size of a lens is enlarged.
Furthermore, in view of the need for a compact imaging apparatus, it is difficult to use a large lens. Additionally, the number of pixels provided for a solid-state image sensor incorporated in an imaging apparatus is rapidly increasing. Therefore, light falloff at edges greatly increases.
A technique capable of downsizing an imaging apparatus while reducing light falloff at edges is discussed in Japanese Patent Application Laid-Open No. 2003-110936.
According to an imaging apparatus discussed in Japanese Patent Application Laid-Open No. 2003-110936, an image plane is divided into a plurality of blocks and a peripheral brightness correction coefficient corresponding to an aperture value is used for each block. Therefore, an appropriate correction can be performed even when the amount of light falloff at edges is not uniform at four corners of a rectangular image plane. As a result, the imaging apparatus can generate an image having an excellent quality without causing any light falloff at edges.
Furthermore, if an imaging apparatus has an optical image-shake correcting function or an electronic image-shake correcting function, the amount of light falloff at a peripheral region of the image plane tends to fluctuate relative to an amount of light at the central region of the image plane.
This problem is significant if the center of a lens deviates from the center of a solid-state image sensor during a shifting operation of the lens performed in the optical image-shake correction.
Therefore, due to the deviation between the centers of optical axes, the amount of light falloff at edges does not become uniform in a peripheral region of the image plane and may be greatly changed according to a shift state of the lens.
For the above-described reasons, when the optical image-shake correction is performed, the occurrence level (frequency) of light falloff at edges changes according to a shift amount of the lens. A similar phenomenon occurs in the electronic image-shake correction, which performs an image shake correction by changing a pixel extraction area.
To address the above-described problems, as discussed in Japanese Patent Application Laid-Open No. 2006-165784, an imaging apparatus is capable of calculating the central position of an optical axis and changing a correction amount for correcting light falloff at edges according to the central position of the optical axis.
As described above, the imaging apparatus discussed in Japanese Patent Application Laid-Open No. 2003-110936 constantly performs a correction of light falloff at edges using a peripheral brightness correction coefficient corresponding to an aperture value for each block during an object shooting operation.
Thus, if the aperture value dynamically changes according to exposure conditions or a zoom operation during an object shooting operation, the correction value for correcting light falloff at edges dynamically changes.
However, before starting the correction after completing calculation of the correction coefficient according to an exposure state during an object shooting operation, an aperture value may change and, therefore, the amount for correction may become excessively large or small. This problem is serious in a shooting operation for a moving image, in which the continuity of displayed video images is essentially required.
According to the imaging apparatus discussed in Japanese Patent Application Laid-Open No. 2006-165784, when a user holds the imaging apparatus to capture a moving image, the image shake correction is constantly performed and, therefore, an image shake correction amount dynamically changes.
Thus, before starting the correction after completing calculation of a correction coefficient according to a calculation result of the optical axis position, the actual position of an optical axis changes if a camera shake is caused during an object shooting operation. Thus, the amount for correction may become excessively large or small.